1. Field of the Invention
The present invention relates to a magnetoresistive film utilizing the magnetoresistance effect, and a magnetic memory using the same. Particularly, the invention concerns a magnetoresistive element and a magnetic memory in which magnetic layers are comprised of perpendicularly magnetized films.
2. Related Background Art
The magnetoresistive films are now used in reproducing heads of hard disk drives and are indispensable to hard disks of high recording density. The fundamental layout of the magnetoresistive films is the sandwich structure in which magnetic layers are contiguously formed through a nonmagnetic layer. In the magnetoresistive film used in the reproducing heads, directions of magnetization are fixed in one magnetic layer. For this reason, it is common practice to let an antiferromagnetic layer exchange-coupled with the magnetic layer for fixing the directions of magnetization and fix the magnetization directions of the magnetic layer along the uniaxial anisotropy of the antiferromagnetic layer. The magnetoresistive film of this film structure is called a spin valve film. A direction of magnetization in the other magnetic layer is reversed in accordance with a direction of a stray field from the hard disk and information recorded in the hard disk is detected based on change in electric resistance of the magnetoresistive film at this time.
Further, research is under way to apply the magnetoresistive film to solid state memories.
In recent years, semiconductor memories being the solid state memories are frequently used in information equipment and there are a variety of types of the semiconductor memories including DRAMs, FeRAMs, flash EEPROMs, and so on. These semiconductor memories have both merits and demerits of characteristics and there exists no memory satisfying all the specifications required in the current information equipment. For example, the DRAM permits high recording density and many rewriting operations, but is volatile and thus loses information without supply of power. The flash EEPROM is nonvolatile, but takes a long time for erasing and is thus not suitable for fast processing of information.
Under the present circumstances of the semiconductor memories as described above, memories making use of the magnetoresistance effect (Magnetic Random Access Memories: MRAMs) are promising as memories satisfying the specifications required in many information devices, including the recording time, reading time, recording density, the number of rewriting operations permitted, power consumption, and so on. The magnetoresistive films are constructed in the structure in which a nonmagnetic film is interposed between magnetic films. Materials often used for the nonmagnetic film are Cu and Al2O3. The magnetoresistive films using a conductor of Cu or the like as the nonmagnetic film are called giant magnetoresistive (GMR) films, and those using an insulator of Al2O3 or the like as the nonmagnetic film are called spin-dependent tunneling magnetoresistive (TMR) films. The TMR films exhibit the greater magnetoresistance effect and are thus advantageous in increase in recording density or in fast readout, and the feasibility thereof as MRAMs is justified in recent research reports.
The electric resistance of the magnetoresistive film is relatively small in a parallel state of magnetization directions in magnetic layers 41 and 42, as shown in FIG. 1A, but the electric resistance is relatively large in an antiparallel state of magnetization directions, as shown in FIG. 1B.
As the device size is decreased in order to increase the recording density of MRAM, the MRAM using longitudinally magnetized films comes to encounter a problem of failure in retaining information, because of influence of a demagnetizing field or influence of curling of magnetization at end faces. This problem can be avoided, for example, by a method of making the magnetic layers in the rectangular shape, but this method does not allow the device size to be decreased to small size, so that little increase can be expected in recording density. There were thus proposals to avoid the above problem through use of perpendicularly magnetized films, for example, as described in U.S. Pat. No. 6,219,275. Since this method does not increase the demagnetizing field even with decrease in device size, it permits the magnetoresistance films to be formed in smaller size than the MRAMs using the longitudinally magnetized films.
The magnetoresistive films using the perpendicularly magnetized films exhibit the following relations between magnetization directions and electric resistances; the electric resistance of the magnetoresistive film is relatively small in a parallel state of magnetization directions in magnetic layers 21, 25, as shown in FIG. 2A; the electric resistance is relatively large in an antiparallel state of magnetization directions, as shown in FIG. 2B.
The magnetoresistive films using the perpendicularly magnetized films are excellent in capability of reducing the device size as described above. The perpendicularly magnetized films include artificial lattice multilayer films of Pt and Co, CoCr alloys, or rare earth-transition metal alloys, etc., and it is preferable to select materials with an aspect ratio of a magnetization curve thereof close to 1, as the magnetoresistive films used in the reproducing heads and the MRAMs; the magnetic materials having such magnetic characteristics include the rare earth-transition metal alloys. These materials, however, had a problem that the rare earth metal existed at the interface to the nonmagnetic layer to impede achievement of the high magnetoresistance effect. Further, the rare earth element is extremely easy to oxidize, and there thus arose a problem that, particularly, when the nonmagnetic layer between the two magnetic layers was an oxide, the rare earth element existing at the interface became oxidized, so as to degrade the magnetic characteristics of the magnetic layers and oxygen existing in the nonmagnetic layer diffused, so as to degrade the magnetoresistance effect.